Fix kernel NULL pointer dereference in xen-blkfront

When booting Xen Dom0 on a pre-release 3.2.1 hypervisor the system Oopses on a
"Unable to handle kernel NULL pointer dereference" in xenwatch.

From the backtrace it looks like backend_changed is calling bdget_disk
with a NULL pointer.  Checking for NULL and returning ENODEV instead
allows the kernel to boot.

1 files changed, 2 insertions, 0 deletions

diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
index b6c8ce254359..8f905089b72b 100644
--- a/drivers/block/xen-blkfront.c
+++ b/drivers/block/xen-blkfront.c
@@ -977,6 +977,8 @@ static void backend_changed(struct xenbus_device *dev,
                 break;
 
         case XenbusStateClosing:
+                if (info->gd == NULL)
+                        xenbus_dev_fatal(dev, -ENODEV, "gd is NULL");
                 bd = bdget_disk(info->gd, 0);
                 if (bd == NULL)
                         xenbus_dev_fatal(dev, -ENODEV, "bdget failed");